In recent years, in the field of continuous casting of metals, various proposals have been made regarding a technique, for casting a thin cast strip having a thickness (2 to 10 mm) close to the thickness of a final product produced by a continuous casting apparatus, using cooling rolls provided with cooling mechanisms inside thereof so as to reduce the production costs, create novel materials, etc.
In the above-described casting technique, the so-called "twin-roll continuous casting process", already known in the art, comprises placing a pair of cooling rolls, rotatable respectively in opposite directions, so as to face each other in parallel while providing a suitable gap therebetween, pressing two side gates against both end faces of the cooling rolls to form a pouring basin for a molten metal above the gap, and continuously casting a thin sheet through the gap, while cooling the molten metal in the pouring basin and rotating the outer periphery of the cooling rolls.
Japanese Unexamined Patent Publication (Kokai) Nos. 60-166149, 63-180348 and 63-183750 and the like disclose a variable width strip casting technique (the twin-roll system) in which casting is effected with the width of a cast strip being arbitrarily varied. Specifically, in a continuous casting machine described in Japanese Unexamined Patent Publication (Kokai) NO. 60-166149, a rotary cooling drum is shifted in the axial direction thereof, and a shield plate fitted onto the surface of the drum is pressed with a spring against the side face of the other drum to form a pouring basin, thereby allowing the width of the cast strip to be varied. Japanese Unexamined Patent Publication (Kokai) No. 63-180348 discloses a casting method wherein casting is effected while vibrating, in the direction of the circumferential direction of the cooling rolls, a side gate provided in contact with the side face of a first cooling roll shifted towards the axial direction of the cooling roll and the circumferential surface of the other cooling roll. Japanese Unexamined Patent Publication (Kokai) No. 63-183750 discloses a side gate which has a tapered end portion to abut against the circumferential surface of the cooling drum to prevent the molten metal from penetrating into the gap between the cooling drum and the side gate.
However, it is difficult to prevent the molten metal from penetrating into the gap between the end face of the cooling roll and the side weir or the gap between circumferential surface of the cooing roll and the side weir (i.e., penetration of molten metal) even by mechanical pressing of the side weir against the cooling roll in one direction or application of vibration to the side weir as in the above-described techniques, which cause the molten metal to penetrate into the above-described gap to form a cast fin on the cast strip. This fin unfavorably shaves the refractory material of the side gate, which causes leakage of the molten metal.
In order to prevent the leakage of the molten metal, Japanese Unexamined Patent Publication (Kokai) No. 62-104653 discloses a technique where an electrode slides on the surface (circumferential surface) of energizable cooling rolls in a twin-roll system to feed DC (direct current) to a molten metal present in the gap between the cooling rolls. Also a DC magnetic flux acts on the molten metal in a direction normal and opposite to the direction of the above-described DC current by means of a DC magnetic flux generator provided in the vicinity of the end portion of each cooling roll to apply electromagnetic force from the end portion of the cooling roll towards the inside of the roll in the axial direction of the roll, thereby holding the molten metal about to leak out from the end portion of the cooling roll and to regulate the shape of the end face of the molten metal.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 62-77154 discloses a technique where an electrode for energizing a molten metal is provided on a supporting shaft of cooling rolls in a twin-roll system to feed current to a molten metal. According to the technique an energizing plate (a side dam) is provided on both outer ends (end faces) of the cooling roll so as to block the molten metal and to feed current to the molten metal in a direction opposite to the direction of the above-described current, thereby generating electromagnetic repulsive force in the molten metal in the vicinity of the energizing plate to prevent the molten metal from leaking out from the side face of the roll.
Further, Japanese Unexamined Patent Publication (Kokai) No. 63-97341 discloses a technique where magnets are provided on the side end face of cooling rolls in a twin-roll system to form magnetic fields which repel each other in the direction of the magnetic lines of force with DC being allowed to flow between an electrode provided in a ladle and a contact provided on a cast metallic sheet, thereby giving rise to an electromagnetic force which holds the molten metal between the cooling rolls.
In the above-described techniques, where electromagnetic force is induced to prevent the leakage of the molten metal, since DC is applied to the whole molten metal, when the space in the horizontal direction between the cooling rolls is large, it becomes difficult for an ordinary electromagnetic force, generated by current and magnetic field strength, to hold the molten metal which in turn makes it difficult to prevent the leakage of the molten metal.
Further, when the molten metal has a height of, for example, 50 mm or more, since the electromagnetic force provided by the above-described methods cannot completely stop vibration caused in a pouring basin portion, the vibration causes the end portion of the resultant cast strip to become wavy, so that the end portion of the cast strip must be cut off in a later step, which reduces the efficiency of the casting machine and the yield of the cast strip.